In the production of gears by the face milling process, tooth slots are formed individually in succession by feeding a rotating tool into a workpiece to a predetermined depth, withdrawing the tool, and indexing the workpiece to the next tooth slot position (intermittent indexing). The steps of feeding, withdrawing and indexing are repeated until all tooth slots are formed. Face milling processes can be carried out as a non-generating process (where the profile shape of a tooth on a workpiece is produced directly from the profile shape on the tool) or a generating process (where the tool and workpiece are rolled together in a predetermined relative rolling motion, known as the generating roll, to “generate” the profile shape of a tooth).
In a finishing process such as grinding, a circular (e.g. cup-shaped) grinding wheel is suitable for finishing the teeth of a workpiece that have been formed by a non-generated or generated face milling cutting process. Grinding processes may also be non-generating or generating. Machines for carrying out such cutting and grinding processes are well known to the skilled artisan with the CNC machine as disclosed in U.S. Pat. No. 6,712,566 being one example of such a commercially available machine.
With use, a grinding wheel becomes worn to the point where it is no longer suitable for producing precise tooth surfaces. Therefore, for dressable grinding wheels, it is customary to periodically dress (renew) a grinding wheel after a specified number of gears (e.g. one or more) have been ground in order to restore the original cutting condition of the grinding wheel.
In order to compensate for the normal continual wear of a grinding wheel during processing, it is known to program a grinding machine to effectively feed the dimensionally-shrinking grinding wheel progressively deeper into successive tooth slots of a gear workpiece by small increments in order to maintain uniform tooth depth and thickness.
It has been observed that with freshly-dressed grinding wheels, the grinding wheel exhibits a higher than normal amount of wear during grinding of the first few (e.g. 1, 2, 3 or more) tooth slots of a workpiece. This is believed due to some residual amount of loose abrasive grains and/or binder material resulting from the dressing process which is then rapidly worn away once grinding commences. In some instances, a freshly-dressed grinding wheel is actually sharper than intended due to the removal of binder material during dressing resulting in some abrasive grains projecting out of the remaining binder material by an excessive amount.
One solution to the above wear condition is addressed in U.S. Pat. No. 7,974,730 wherein it is disclosed that when grinding with a freshly-dressed grinding wheel, an initial number of tooth slots (e.g. 1, 2 or 3) are pre-machined to a depth that is considerably less than full-depth in order to overcome the initial rapid wear of the grinding wheel, the remainder of the tooth slots are ground normally and then the pre-machined slots are post-machined (e.g. regrinding) to achieve the appropriate depth. This process, however, requires additional cycle time in order to twice grind a certain number of tooth slots.